December 1, 2004
12:01 AM (EST)

News Release Number: STScI-2004-35

Hubble Uncovers a Baby Galaxy in a Grown-Up Universe

December 1, 2004: Scientists using NASA's Hubble Space Telescope have measured the age of
what may be the youngest galaxy ever seen in the universe. By
cosmological standards it is a mere toddler seemingly out of place among
the grown-up galaxies around it. Called I Zwicky 18, it may be as
young as 500 million years old (so recent an epoch that complex life
had already begun to appear on Earth). Our Milky Way galaxy by
contrast is over 20 times older, or about 12 billion years old, the
typical age of galaxies across the universe. This "late-life" galaxy
offers a rare glimpse into what the first diminutive galaxies in the
early universe look like.

Q & A: Understanding the Discovery

1.
How can a 500-million-year-old galaxy be considered young?

The universe is about 14 billion years old. So, when you compare the
galaxy with the age of the universe, it is a relative youngster. In
fact, if you could compress the evolution of the universe into a 24-hour
day, most of the galaxies started forming at 1 a.m. and were fully grown
before daybreak. The galaxy I Zwicky 18 did not start forming until 11 p.m.
in the evening!

2.
How do astronomers know the galaxy is young?

Astronomers know the galaxy is a youngster because they don't see any
nomal stars older than 500 million years. Even more conservative
estimates nudge the oldest stars to 1 billion years. The galaxy has gone
though several sudden bursts of star formation, the latest only 4 million
years ago. Also, measuremants show that the galaxy has a paucity of heaver
elements beyond helium. This means there have been no successive generations
of stars to manufacture the heavier elements through nuclear fusion.

3.
Why did it take so long for the galaxy to make stars?

This is quite a mystery with no obvious answer. The galaxy is sort of a Rip
Van Winkle that has remained quiescent as the rest of the universe evolved
around it. A chance close encounter with another dwarf galaxy may have at
last jump-started it into making stars. Gravitational perturbations between
the galaxies may have caused hydrogen clouds to collapse and precipitate
stars.